Obesity and diabetes are among the most common human health problems in industrialized societies. Obesity, which is the result of an imbalance between caloric intake and energy expenditure, is highly correlated with insulin resistance and diabetes in experimental animals and humans. However, the molecular mechanisms that are involved in obesity-diabetes syndromes are not clear. Since adipose tissue is the major site for energy storage and mobilization, many investigators have focused on finding abnormalities in adipocyte physiology or metabolism (Plata-Salaman, Brain Behav. Immun. 3:193, 1989; Lardy et al., Annu. Rev. Biochem. 59:689, 1990).
It has been shown that several cytokines such as tumor necrosis factor (TNF)-.alpha. have direct effects on adipocyte metabolism as well as other important metabolic actions (Le et al., Lab. Invest. 56:234, 1987; Dinarello, Immunol. Lett. 16:227, 1987; Kunkel et al., Crit. Rev. Immunol. 9:93, 1989; Grunfeld et al., Biotherapy 3:143, 1991). TNF-.alpha. acts in vitro on murine adipocytes to suppress expression of most adipose specific genes including enzymes involved in lipogenesis (Kawakami et al., Proc. Natl. Acad. Sci. USA 79:912, 1982; Price et al., Arch. Biochem. Biophys. 251:738, 1986). However, some of these effects are not observed in primary cultures of human or rat adipocytes (Grunfeld et al., Biotherapy 3:143, 1991; Kern, J. Lipid Res. 29:909, 1988).
In vivo, TNF-.alpha. expression has been associated with catabolic states leading to a "wasting syndrome," termed cachexia (Beutler et al., Nature 316:552, 1985; Beutler et al., Science 232:977, 1986; Beutler et al., Nature 320:584, 1986; Oliff et al., Cell 50:555, 1987; Beutler et al., Ann. Rev. Immunol. 7:625, 1989), but this effect of TNF-.alpha. has been challenged by several groups of investigators (Semb et al., J. Biol. Chem. 262:8390, 1987; Grunfeld et al., J. Lipid Res. 30:579, 1989; Feingold et al., J. Clin. Invest. 83:1116, 1989; Patton et al., J. Clin. Invest. 80:1587 (1987); Kettlehut et al., J. Clin. Invest. 81:1384, 1988; Tracey et al., J. Clin. Invest. 86:2014, 1990; Socher et al., J. Exp. Med. 167:1957, 1988; Mullen et al., Proc. Soc. Exp. Biol. Med. 193:318, 1990; Teng et al., Proc. Natl. Acad. Sci. USA 88:3535, 1991; for reviews see C. Grunfeld et al., Cancer Res. 49:2554, 1989; Fiers, FEBS 285:199, 1991).
TNF-.alpha. administration causes an increase in serum triglycerides and very low density lipoproteins in rats and humans (Semb et al., J. Biol. Chem. 262:8390, 1987; Grunfeld et al., J. Lipid Res. 30:579, 1989; Feingold et al., J. Clin. Invest. 83:1116, 1989; Sherman et al., J. Clin. Oncol. 6:344, 1988). This hyperlipidemia is thought to be the result of decreased lipoprotein lipase activity and increased hepatic lipogenesis (Feingold et al., J. Clin. Invest. 80:184, 1987). TNF-.alpha. administration also has effects on appetite and gastrointestinal tract functions (Plata-Salaman, Brain Behav. Immun. 3:193, 1989). Besides TNF-.alpha., other cytokines such as TNF-.beta., IL-1, IL-6 and interferon (INF) also have profound effects on lipid metabolism (Grunfeld et al., Biotherapy 3:143, 1991). Furthermore, all of these cytokines affect glucose homeostasis in various tissues (Rey et al., Am. J. Physiol. 253:R794, 1987; Meszaros et al., Biochem. Biophys. Res. Comm. 149:1, 1987; Koivisto et al., Diabetes 38:641, 1989; Snick, Annu. Rev. Immunol. 8:253, 1990).
Previous studies have also suggested an association of TNF-.alpha. with states of peripheral insulin resistance, especially in infection. First, it is established that biological mediator(s) generated during infection interfere with insulin's actions and lead to profound metabolic alterations (Beutler et al., Ann. Rev. Immunol. 7:625, 1989; Stephens et al., J. Biol. Chem. 266:21839, 1991; Beisel, Ann. Rev. Med. 26:9, 1975; Stephens et al., Biochem. Bioph. Res Common. 183:417, 1992). Second, incorporation of glucose into lipids is decreased upon short term treatment of 3T3-L1 cells with supernatants of activated macrophages (Olney, Science 164:719, 1969; Cameron et al., Cli. Exp. Pharmacol. Physiol. 5:41, 1978), and third, treatment of L6 myotubes (Cornelius et al., J. Biol. Chem. 265:20506, 1990) and 3T3-L1 adipocytes with recombinant TNF-.alpha. causes downregulation of Glut4 expression (Stephens et al., J. Biol. Chem. 266:21839, 1991). However, the specificity of TNF-.alpha.'s effect on Glut4 mRNA in fat cells was not clear in that expression of many or most other fat cell genes was also affected (Stephens et al., J. Biol. Chem. 266:21839, 1991). Finally, a recent study has directly demonstrated that chronic, low level administration of TNF-.alpha. to rodents induces systemic insulin resistance (Lang et al., Endocrinology 130:43, 1992).
Insulin resistance, defined as a smaller than expected biological response to a given dose of insulin, is a ubiquitous correlate of obesity. Indeed, many of the pathological consequences of obesity are thought to involve insulin resistance. These include hypertension, hyperlipidemia and, most notably, non-insulin dependent diabetes mellitus (NIDDM). Most NIDDM patients are obese, and a very central and early component in the development of NIDDM is insulin resistance (reviewed in Moller et al., New Eng. J. Med. 325:938, 1991). It has been demonstrated that a post-receptor abnormality develops during the course of insulin resistance, in addition to the insulin receptor downregulation during the initial phases of this disease (Olefsky et al., in Diabetes Mellitus, H. Rifkin and D. Porte, Jr., Eds. (Elsevier Science Publishing Co., Inc., New York, ed. 4, 1990), pp. 121-153). Several studies on glucose transport systems as potential sites for such a post-receptor defect have demonstrated that both the quantity and function of the insulin sensitive glucose transporter (Glut4) is deficient in insulin resistant states of rodents and humans (Garvey et al., Science 245:60, 1989; Sivitz et al., Nature 340:72, 1989; Berger et al., Nature 340:70, 1989; Kahn et al., J. Clin. Invest. 84:404, 1989; Charron et al., J. Biol. Chem. 265:7994, 1990; Dohm et al., Am. J. Physiol. 260:E459, 1991; Sinha et al., Diabetes 40:472, 1991; Friedman et al., J. Clin. Invest. 89:701, 1992). A lack of a normal pool of insulin-sensitive glucose transporters could theoretically render an individual insulin resistant (Olefsky et al., in Diabetes Mellitus, H. Rifkin and D. Porte, Jr., Eds. (Elsevier Science Publishing Co., Inc., New York, ed. 4, 1990), pp. 121-153). However, some studies have failed to show downregulation of Glut4 in human NIDDM, especially in muscle, the major site of glucose disposal (for a review see G. I. Bell, Diabetes 40:413, 1990; Pederson et al., Diabetes 39:865, 1990; Handberg et al., Diabetologia 33:625, 1990; Garvey et al., Diabetes 41:465, 1992).
The mechanistic link between obesity and insulin resistance is not understood. Much attention has been focused on the role of free fatty acids as potential mediators of insulin resistance (Reaven et al., Am. J. Med. 85:106, 1988; Lonnroth, J. Intern. Med. Suppl. 735:23, 1991; Bjorntorp, Diabetes Care 14:1132, 1991). Free fatty acid levels are typically elevated in obesity, and fatty acids have been shown to affect insulin sensitivity in vitro and in vivo (Reaven et al., Am. J. Med. 85:106, 1988; Lonnroth, J. Intern. Med. Suppl. 735:23, 1991; Bjorntorp, Diabetes Care 14:1132, 1991).